| ZA27 alloy is widely used for its good mechanical properties and high wear resistance. However, due to the serious dendritic segregation of solidification structure, the defects of serious shrinkage cavity and porosity of castings and its poor high-temperature friction properties, ZA27 alloy’s application is limited in high-temperature environment.To improve the mechanical properties of ZA27 alloy, this thesis focused on preparing SiC/ZA27 composites by using semi-solid extrusion method. At the same time, the paper studied the functions of semi-solid formation and heat treatment on the structure and properties of composites. Related results were shown as follows:(1)The combination of the semi-solid extrusion molding method with semi-solid billets can break the dendrite grain, refine particles and disperse SiC particle aggregates, which achieved a more uniform particle distribution and enhances the bonding strength between the particles and the matrix.Compared with cast molding, the properties of SiC/ZA27 composites prepared by semi-solid extrusion method had improved significantly.(2)Optimum extrusion temperature for preparing SiC/ZA27 composite was 475℃.At this temperature solid-liquid phase ratio, melt viscosity and flow were moderate.The prepared composite had excellent structure with fine grains and dense tissue, the distribution of particles aggregate was fine point, short-like,the density was 4.8521g/m3;the mechanical properties were also improved,the hardness 148HB, yield strength 208.35MPa, tensile strength 318.73MPa, elongation was 3.36%.(3)The best pressing force prepared SiC/ZA27 composite was 15MPa. The effect of strong squeezing pressure compression enhanced the melt flow,crushing ability, therefore the structure was fine grain and SiC particles uniformly distributed.The density was 5.0849g/cm3,hardness was158HB,yield strength was 3.87MPa,tensile strength was 335.56MPa, elongation was 3.79%.(4)Prepared SiC/ZA27 composite with different SiC particle contents, the more the contents of SiC particles, the higher the viscosity,the worse the flow-ability, which reduced particle dispersion, tissue density and particle strengthening effect. Optimum mechanical properties of the particle mass fraction is 10%.(5)After T5 heat treatment,10wt% SiC/ZA27 composites reduced the brittleness of the non-equilibrium (α’+η+ε) eutectic phase, obtained enhanced (a+η) two-phase structure,precipitated Mg32Al47Cu7 fine phase in the matrix. After the heat treatment, the composite was still mixed fracture mechanism between ductile and cleavage, but ductile fracture mechanism was more obvious.(6)The heat treatment promoted the strength, toughness and bonding strength of particles and matrix of 10wt% SiC/ZA27 composites, thus the mechanical properties significantly improved. Compared with the composite without the heat treatment,The hardness 188HB,improved 19.21%, yield strength 234.52MPa, improved 9.66%, the tensile strength as 352.11MPa,improved 4.93%,the elongation 4.16%,improved 9.76%.After heat treatment,the amount of wear reduced significantly,the wear resistance improved significantly.(7)After heat treatment,the 10wt%SiC/ZA27 composite extruded in the extrusion temperature of 47℃ and extrusion pressure of 15MPa,got the optimal organization and performance. |
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